A noise suppressing technology is known as a signal processing technology of suppressing noise in a noisy signal (a signal containing a mixture of a target signal and noise) partially or entirely and outputting an enhanced signal (a signal obtained by enhancing a target signal). For example, a noise suppressor is a system that suppresses noise superposed on a target audio signal. The noise suppressor is used in various audio terminals such as mobile phones.
Concerning technologies of this kind, patent literature 1 discloses a method of suppressing noise by multiplying an input signal by a suppression coefficient smaller than 1. Patent literature 2 discloses a method of suppressing noise by directly subtracting estimated noise from a noisy signal.
The techniques described in patent literature 1 and 2 need to estimate noise from the target signal that has already become noisy due to the mixed noise. However, there are limitations on accurately estimating noise only from the noisy signal. Hence, generally, the methods disclosed in patent literature 1 and 2 are effective only when noise is much smaller than a target signal. If the condition that the noise is sufficiently smaller than the target signal is not satisfied, accuracy of a noise estimation value is poor. For this reason, the methods disclosed in patent literature 1 and 2 can achieve no sufficient noise suppression effect, and the enhanced signal includes a large distortion.
On the other hand, patent literature 3 discloses a noise suppression system that can realize enough noise suppression effect and small distortion in the enhanced signal even if the condition that the noise is sufficiently smaller than the target signal is not satisfied. Assuming that the characteristics of noise to be mixed into the target signal are known to some extent in advance, the method disclosed in patent literature 3 suppresses the noise by subtracting noise information (information about the noise characteristics) which is recorded in advance from the noisy signal. Patent literature 3 also discloses a method of multiplying the noise information by a large coefficient if input signal power obtained by analyzing an input signal is large; or a small coefficient if the input signal power is small, and subtracting the multiplication result from the noisy signal.